Effects of Insulin on the Circadian Rhythm of Voluntary Locomotor Activity in Rats

Experiments on male Wistar rats were performed to study the effects of single intranasal doses of 0.2 mg of insulin at different times in the projected daily cycle (ZT = 1, ZT = 7, ZT = 13, and ZT = 19 h) on the circadian rhythm of voluntary locomotor activity in a running wheel. Insulin given at ZT = 7 and ZT = 13 h induced statistically significant phase shifts in the rhythm, with advancement by 4.4 and 5.5 h, respectively. Administration of insulin at ZT = 13 h also induced shortening of the period of the circadian rhythm of voluntary locomotor activity. Intranasal insulin at other time points of the projected daily cycle (ZT = 1 or ZT = 19 h) did not lead to statistically significant phase shifts or changes in the period of the circadian rhythm. Insulin produced no changes in the total daily activity, regardless of the administration time. These results point to a possible role for endogenous insulin as a factor tuning the circadian oscillator in the absence of a fundamental physiological zeitgeber – cyclic afferentation from retinal photoreceptors.